<!DOCTYPE html>
<html lang="zh-hans">
<head>
  <meta charset="UTF-8">
  <title>加法器</title>
</head>
<body>
  <input type="text" id="a">
  <!-- <span>+</span> -->
  <select id="op">
    <option value="+">+</option>
    <option value="-">-</option>
    <option value="*">*</option>
    <option value="/">/</option>
  </select>
  <input type="text" id="b">
  <button type="button">=</button>
  <span id="result"></span>
  <script src="adder.js"></script>
</body>
</html>